Polarizers used in liquid crystal display devices (LCDs) generally have a structure in which a polarizing film obtained by dyeing polyvinyl alcohol (PVA), etc., with iodine is sandwiched between two protective films. Triacetyl cellulose (TAC) films are commonly used as the protective films. Along with the recent trend toward thinner LCDs, there is a demand for reducing the thickness of the polarizers. However, when the thickness of a TAC film is reduced, problems such as lower mechanical strength and higher moisture permeability occur. Moreover, since TAC films are relatively expensive, inexpensive alternative materials are strongly desired.
To address this situation, there is a proposal to use polyester films, which have relatively high durability despite their thinness, in place of TAC films (PTL 1 to PTL 3).
TAC films, the surface of which is treated with alkali, etc., have a very high affinity for hydrophilic adhesives and have very high adhesion to polarizing films coated with a hydrophilic adhesive. In contrast, polyester films have insufficient adhesion to hydrophilic adhesives. In particular, this tendency is more prominent in polyester films having orientation due to a stretching treatment. Accordingly, PTL 2 and PTL 3 propose providing an easy-bonding layer in a polyester film in order to improve the adhesion of the polyester film to a polarizing film or a hydrophilic adhesive applied to the polarizing film.